1. Field of the Invention
The present invention relates to small- and thin-sized variable resistors, and in particular relates to a holding structure for metallic terminals by a substrate.
2. Description of the Related Art
As disclosed in Japanese Unexamined Patent Application Publication No. 7-86001, there is known a conventional variable resistor wherein a first metallic terminal having an eyelet on a substrate is formed by insert-molding while a second metallic terminal is formed by insert-molding; a substantial arc resistor conducting the second metallic terminal is formed on the top surface of the substrate; then a slider including a drawn portion having a hole formed in the center thereof is fitted into the eyelet of the first metallic terminal and attached to the substrate by crimping the eyelet outwardly. The slider is unitarily formed of a contact arm portion sliding on the resistor and an adjusting portion to be rotationally operated with a tool.
The structure of the variable resistor described above is that external-connecting portions of the first and second metallic terminals are extended from the center portions of the substrate side walls in the thickness direction to be bent toward the bottom surface of the substrate. Thereby, the product can be reduced in size, and also enabling discrimination of the product when it is soldered to a printed board to be readily performed.
In order to achieve miniaturizing and reduction in thickness of such a variable resistor, there are methods of reduction in thickness of the substrate disposed in both sides of the insert-molded first and second metallic terminals; and of elimination of the substrate located in the bottom surface side lower than the terminal. A variable resistor made by the latter method is disclosed in Japanese Unexamined Patent Application Publication No. 9-223608. In this case, the external-connecting portions of the first and second metallic terminals are extended from the bottom surface of the substrate to be bent upwardly along side faces of the substrate.
Although miniaturization and reduction in the thickness of the variable resistor can be achieved when any of the methods described above is used, the holding strength for metallic terminals by the substrate decreases. Also, when the external-connecting portion of the metallic terminal is subjected to bending work, the bending stress is concentrated in the substrate portion supporting the external-connecting portion. When the thickness of the substrate is reduced as described above, rattles may be likely generated in the metallic terminal because of the reduced holding strength for the terminal. In particular, one end of the second metallic terminal is extended to be exposed to the substrate top face; the exposed portion is electrically connected to the resistor formed on the top face of the substrate; when this metallic terminal rattles, electrical connection to the resistor becomes unstable, resulting in deterioration of characteristics.
Accordingly, it is an object of the present invention to provide a thin-thickness variable resistor capable of stabilizing electrical connection between the metallic terminals and the resistor by increasing the holding strength for the second terminal by the substrate while preventing the bending strain of the terminal during the bending work from spreading to the conductive portion conductive to the resistor.
In order to achieve the above-mentioned object in accordance with a first aspect of the present invention, there is provided a variable resistor comprising: a substrate comprising a first metallic terminal formed in the substrate by insert-molding; a substantially arc-shaped resistor formed on the top surface of the substrate; and a second metallic terminal formed in the substrate by insert-molding and having a conductive portion conducted to the resistor; and a slider connected to the first metallic terminal and comprising a contact arm portion sliding on the resistor and an adjusting portion to be rotationally operated with a tool, wherein the slider is rotatably attached to the substrate, and wherein external-connecting portions of the first and second metallic terminals are extended from the substrate; anchor portions to be embedded into the substrate are integrally formed in the vicinity of the conductive portion of the second metallic terminal, and the anchor portions are integrally formed between the conductive portion of the second metallic terminal and the external connection portion thereof.
With these features, by unitarily forming anchor portions to be embedded into the substrate in the vicinity of the conductive portion of the second metallic terminal, and between the conductive portion of the second metallic terminal and the external connection portion thereof, the holding strength for the second metallic terminal by the substrate can be increased. In other words, at the portion where the terminal should not move as the function of the member and in the vicinity of the point to which an external stress is applied, the holding strength by which the substrate holds the second metallic terminal firmly can be increased to stabilize the electrical connection between the conductive portion of the second metallic terminal and the resistor, and avoid that the metallic terminal is disconnected inadvertently.
Preferably, in the variable resistor, the external-connecting portions of the first and second metallic terminals are bent along sides of the substrate.
In this case, since the anchor portion also has the effect of restraining the strain during the bending work of the external-connecting portion from spreading to the conductive portion, the electrical connection between the second metallic terminal and the resistor can be always stabilized.
Also, when the variable resistor is soldered to a printed board, since fillet is formed between the external-connecting portion and the printed board by bending the external-connecting portion of the metallic terminal along sides of the substrate, whether the soldering is sufficient or not can be easily judged by appearance.
In accordance with a second aspect of the present invention, there is provided a variable resistor comprising: a substrate comprising a first metallic terminal formed in the substrate by insert-molding and having an eyelet in the center thereof; a substantially arc-shaped resistor formed on the top surface of the substrate concentrically with the eyelet; and a second metallic terminal formed in the substrate by insert-molding and having a conductive portion conducting the resistor; and a slider comprising a drawn portion having a hole in the center thereof; a contact arm portion disposed in the periphery of the drawn portion and sliding on the resistor; and an adjusting portion to be rotationally operated with a tool, wherein the hole of the drawn portion is fit into the eyelet of the first metallic terminal, and the slider is rotatably attached to the substrate by crimping the eyelet to be spread outwardly, and wherein external-connecting portions of the first and second metallic terminals are extended from the bottom portion of said substrate; anchor portions to be embedded into said substrate are integrally formed in the vicinity of the conductive portion of the second metallic terminal, and the anchor portions are integrally formed between the conductive portion of the second metallic terminal and the external connection portion thereof.
By extending the external-connecting portions of the first and second metallic terminals from the bottom portion of the substrate in such a manner, the thickness of the substrate can be reduced to be smaller compared with that when extending them from the center portion in the thickness direction of the substrate. That is, the overall thickness of the variable resistor can be reduced to be smaller.
Furthermore, in the variable resistor according to the present invention, the external-connecting portions of the first and second metallic terminals may be bent upwardly along sides of the substrate.
In this cases spreading of the bending stress to the conductive portions during the bending work of the external-connecting portion is restrained by the anchor portions. Also, by bending the external-connecting portion of the metallic terminal upwardly from the bottom portion of the substrate along sides of the substrate, the discrimination of the soldering can be readily performed.
The anchor portions are available in various shapes. For example, there is a method to provide a hole or a raised piece by cutting the midway part of the metallic terminal; however, the anchor portion may be a projecting piece bent at an end of the anchor portion. In this case, the anchor portion can be simply formed and takes the secure anchor effect. That is, the second metallic terminal is generally formed by punching a planar metallic plate to have a predetermined shape by a press while being bent; when the anchor portion is the projecting piece having the bent end as described above, it has only the anchor effect without damaging the strength of the metallic terminal while it can be readily formed by an ordinary press.